1. Field of the Invention
This invention relates to the clamping of concentric well casings, where an inner well casing is to be clamped in position relative to an outer well casing, to achieve a desired relative axial position between the casings, for operational reasons axial fixing between coaxial well casings over a range of positions may be required at various times during drilling and/or production from wells, and the present invention makes it possible to clamp one or more casings within another at any desired position and subsequently to unclamp the casings for disassembly, or to change their relative positions and then reclamp the casings in a new relative position. As the clamping mechanism is preinstalled and can be externally activated, the blowout preventers can remain in place throughout the installation, clamping or release of the subsequently installed casing.
2. Description of the Prior Art
In oil and gas wells, it is conventional to pass a number of concentric tubes or casings down the well. An outermost casing is fixed in the ground, and the inner casings are each supported in the wellhead or in the next outer casing by casing or tubing hangers.
These casing hangers may take the form of a body with interengaging internal shoulders on the outer casing and a body with external shoulders on the inner casing hangers, located at fixed positions on each previously installed casing.
There are however applications where a fixed position casing hanger is unsatisfactory, because the hang-off point of one casing on another may require to be adjustable.
The invention has particular application for such casing and tubing hangers, which require adjustment.
Where drilling or production wellheads have to accommodate a casing or tubing without predetermined hang-off point, it has been known to use casing slip-type support mechanisms.
It is also known from European patent number EP251595B2 to use an adjustable landing ring on a surface casing hanger to accommodate a space-out requirement.
It is furthermore known that where production wellheads have to accommodate casing or tubing with a tension load greater than the running weight, retractable shoulders or internal wedge mechanisms have been used to allow passing of the casing or tubing hanger, and re-tensioning to a predetermined point.